Advances in communications technologies, combined with the widespread adoption of personal communication devices (“PCSs”), have revolutionized the way information is disseminated and shared. Information can now be delivered directly to computer desktops, laptops, personal digital assistants (“PDAs”), cellular telephones, digital music players, and other portable devices over wired or wireless connections, providing a virtually unlimited connection experience for all users. In particular, the rapid expansion of wireless technologies has fueled the demand for faster and more efficient wireless transmission of voice, data, and video on a global basis.
Information is transmitted over a wireless channel from an information source to a destination by means of a wireless communications system, such as the conventional system shown in FIG. 1. At its simplest form, wireless communications system 100 includes: (1) modulator 105; (2) transmitter 110; (3) wireless channel 115; (4) receiver 120; and (5) demodulator 125. Modulator 105 processes the information into a form suitable for transmission over wireless channel 115. The information may be in the form of voice, data, audio, imagery, video, or any other type of content conveyed in an information signal, also referred to as a message signal. Modulator 105 essentially translates the message signal into a modulated signal suitable for transmission over wireless channel 115 by modifying one or more characteristics of a carrier signal. The modulated signal is passed on to wireless channel 115 by transmitter 110, which usually filters and amplifies the modulated signal prior to its transmission. The function of wireless channel 115 is to provide a wireless link or connection between the information source and destination. Once transmitted through wireless channel 115, the modulated signal is detected and amplified by receiver 120 to take into account any signal attenuations introduced during transmission by wireless channel 115. Finally, the transmitted signal is demodulated by demodulator 125 so as to produce a close estimate of the original message signal.
The performance of a wireless communications system such as wireless communications system 100 is, in part, dictated by the performance of its modulator, e.g., modulator 105. For it is the modulator that is responsible for converting the message signal into a signal suitable for transmission so as to maximize the use of the overall system resources. For example, a high performance modulator should generate a modulated signal having a frequency spectrum that, when filtered and transmitted by a transmitter such as transmitter 110, would utilize only a small fraction of the total channel bandwidth, thereby enabling many users to share the channel bandwidth simultaneously. A high performance modulator should also work in conjunction with a high performance filter in the transmitter to ensure optimal preservation of the frequency spectrum of the modulated signal.
Current wireless communications systems use many different modulation techniques, including, but not limited to, amplitude shift keying (“ASK”), frequency shift keying (“FSK”), binary phase shift keying (“BPSK”), quadrature phase shift keying (“QPSK”) and its variations, minimum shift keying (“MSK”) and Gaussian minimum shift keying (“GMSK”), among others. These modulation techniques are digital techniques in which the message signal is represented by a sequence of binary symbols. Each symbol may have one or more bits, depending on the modulation technique used.
Typically, these modulation techniques switch or key the amplitude, frequency, and/or phase of a carrier signal according to the binary symbols in the message signal, e.g., according to binary symbols “0” and “1.” For example, different amplitudes are used to represent both binary symbols in ASK, different frequencies are used to represent both binary symbols in FSK, and different phases are used to represent both binary symbols in BPSK. QPSK is a variation of BPSK in which two bits or more are used per symbol. The phase of the carrier takes on one of four equally spaced values, such as 0, π/2, π, and 3π/2, with each value corresponding to a unique symbol, e.g., 00, 10, 11, and 01. MSK and GMSK are variations of FSK in which the change in carrier frequency from one binary symbol to another is half the bit rate of the message signal.
The selection of a digital modulation technique for use in a wireless communications system depends on several factors. A desirable digital modulation technique provides low bit error rates at low signal-to-noise ratios, occupies a minimum bandwidth, performs well in the presence of multipath and fading conditions, and is cost-effective to implement. Depending on the physical characteristics of the channel, required levels of performance and target hardware trade-offs, some modulation techniques will prove to be a better fit than others. Consideration must be given to the required data rate, acceptable level of latency, available bandwidth, and target hardware cost, size, and power consumption. For example, in personal communication systems that serve a large subscriber community, the cost and complexity of the receivers must be minimized. In this case, a modulation technique that is simple to detect is most attractive. In cellular systems where intersymbol interference is a major issue, the performance of the modulation technique in an interference environment is extremely important.
The performance of a modulation technique is often measured in terms of its power efficiency and bandwidth efficiency. Power efficiency describes the ability of a modulation technique to preserve the fidelity, i.e., an acceptable bit error probability, of the message signal at low power levels. In digital communication systems, higher fidelity requires higher signal power. The amount by which the signal power should be increased to obtain a certain level of fidelity depends on the type of modulation employed. The power efficiency of a digital modulation technique is a measure of how favorably this tradeoff between fidelity and signal power is made, and is often expressed as the ratio of the signal energy per bit to noise power spectral density required at the receiver input for a certain probability of error.
Bandwidth efficiency describes the ability of a modulation technique to accommodate data within a limited bandwidth. In general, increasing the data rate implies decreasing the pulse-width of a digital symbol, which increases the bandwidth of the signal. Bandwidth efficiency reflects how efficiently the allocated bandwidth is utilized. Bandwidth efficiency is defined as the ratio of the throughput data rate per Hertz in a given bandwidth. The system capacity of a digital modulation technique is directly related to the bandwidth efficiency of the modulation technique, since a modulation technique having a greater bandwidth efficiency will transmit more data in a given spectrum allocation.
In general, modulation techniques trade bandwidth efficiency for power efficiency. For example, FSK is power efficient but not as bandwidth efficient and QPSK and GMSK are bandwidth efficient but not as power efficient. Since most wireless systems are bandwidth limited due to frequency spectrum allocations, modulation techniques that concentrate their performance on bandwidth efficiency are generally preferable. In fact, most wireless communication standards available today use more bandwidth-efficient modulation techniques such as QPSK and its variations, in use by the PHS and PDC Japanese standards, and IS-54 and IS-95 American standards, and GMSK, in use by the GSM global standard.
The bandwidth efficiencies achieved by the digital modulation techniques currently adopted by the wireless standards are, however, only in the order of 1-10 bps/Hz. Such bandwidth efficiencies are not able to satisfy the rapidly rising demand for faster and more efficient wireless services that are capable of serving a large number of users simultaneously.
To address these concerns, two new sets of modulation techniques have been developed: (1) spread spectrum modulation techniques; and (2) narrowband modulation techniques. Spread spectrum modulation is a technique in which the modulated signal bandwidth is significantly wider than the minimum required signal bandwidth. Bandwidth expansion is achieved by using a function that is independent of the message and known to the receiver. The function is a pseudo-noise (“PN”) sequence or PN code, which is a binary sequence that appears random but can be reproduced in a deterministic manner by the receiver. Demodulation at the receiver is accomplished by cross-correlation of the received signal with a synchronously-generated replica of the wide-band PN carrier.
Spread spectrum modulation has many features that make it particularly attractive for use in wireless systems. First and foremost, spread spectrum modulation enables many users to simultaneously use the same bandwidth without significantly interfering with one another. The use of PN codes allows the receiver to separate each user easily even though all users occupy the same spectrum. As a result, spread spectrum systems are very resistant to interference, which tends to affect only a small portion of the spectrum and can be easily removed through filtering without much loss of information. Additionally, spread spectrum systems perform well in the presence of multipath fading and Doppler spread.
The main disadvantage of spread spectrum systems is that they are very bandwidth inefficient for a single user or a single wireless cell, since the bandwidth utilized is much more than that necessary for transmission. In fact, bandwidth efficiency for a single user is so low that most spread spectrum systems report bandwidth efficiency for the whole channel, to emphasize their ability to simultaneously serve many users with the available channel bandwidth. In addition, spread spectrum systems are also much more complex than systems employing traditional modulation techniques, thereby increasing overall system design, deployment, and maintenance costs.
Narrowband modulation techniques, such as those described in U.S. Pat. No. 5,930,303, U.S. Pat. No. 6,748,022, and U.S. Pat. No. 6,445,737, provide an entirely different approach. Instead of spreading the signal over a wide bandwidth range to optimize the number of users sharing the channel bandwidth simultaneously, narrowband modulation techniques attempt to squeeze the frequency spectrum into a as narrow of a band as possible in order to maximize both the bandwidth efficiency for an individual user and the overall channel utilization for a large number of users. In the narrowband modulation techniques described therein, phase reversals occurring before, in the middle, at the end, or after a bit period are used to generate a modulated signal having most of its energy concentrated in a very narrow peak centered at a carrier frequency. As most of the signal energy is concentrated in the narrow peak, transmission of the modulated signal may be accomplished by transmission of the narrow peak, thereby significantly improving the bandwidth efficiency for an individual user.
While achieving bandwidth efficiencies of 30-60 bps/Hz, these narrowband modulation techniques are not very practical because they require the use of a specialized analog crystal filter with a resonant frequency tuned by a shunt capacitor. Such a filter is very difficult to implement in practice due to tuning imperfections of the shunt capacitor, irregularities of the crystal material employed, and other challenges associated with designing high-precision analog crystal filters. Furthermore, these narrowband modulation techniques are very susceptible to intersymbol interference, may not perform well under high bit error rates, and require higher transmission power than traditional modulation techniques such as FSK and BPSK.
Because currently-available modulation techniques have not been able to achieve high bandwidth and power efficiencies while performing well under various channel conditions, broadband wireless services that reach a large number of users simultaneously have not yet been fully deployed. Such services should be able to serve users with voice, data, audio, imagery, and video at high data rates and low infrastructure costs to service providers and consumers alike. Such services should also be able to optimize the number of users served by better utilization of the allocated frequency spectrum.
In view of the foregoing, there is a need in this art for a digital modulation technique that achieves very high bandwidth efficiency under various channel conditions.
There is a further need in this art for a high-precision narrowband digital filter for use in conjunction with a narrowband digital modulation technique in a communications system that achieves high bandwidth efficiency when transmitting information through narrowband communication channels.
There is also a need in this art for a communications system that optimizes bandwidth utilization when providing wireless services to a large number of users simultaneously under various channel conditions.